Processing of selenium-containing metallurgical dusts and fumes



NOV 14, 1961 E. A. GoDAT, JR., ET AL 3,008,806

PROCESSING OF SELENIUM-CONTAINING METALLURGICAL DusTs AND FUMEs Filed April 15, 1959 coNvlTnoNmz.

INVENTORS l. s. au INTANI LLA, J2.

and.- l.. A. AcvvAT T TOR/YE Y tates nited This invention relates to the processing of seleniumcontaining materials and relates more particularly to an improved process for upgrading the metallurgical dusts or fumes of low selenium content to a point Where selenium recovery from such material is made economically practical.

The low selenium content of such materials renders impractical the application of the conventional procedures for selenium recovery therefrom utilizing for example, the well-known'soda ash roasting process or selenium volatilization methods. For satisfactory results, such procedures require a feed material of appreciably higher selenium content .than normally exists in the fumes or dust obtained from `the gases produced in lead ore smelting or other metallurgical operations that result in production of a selenium-containing dust or fume. Accordingly, the low selenium content material must rst be upgraded to increase .the selenium concentration to a level that makes the processing for selenium recovery feasible. Satisfactory upgrading of dusts Iand fumes for lthe stated purpose .is rather difficult to Iachieve in a completely satisfactory manner and because of this selenium recovery from such materials is not carried out resulting in the loss of the selenium values there-in.

It is lthe principal object of this invention to provide-an improved process for concentrating the selenium values in a low-grade metallurgical dust or fume to enable 'the subsequent recoveryA of said selenium in an economic and practicable manner.

It Iis another object of this invention to provide an improved process for concentrating the selenium content in a metallurgical dust or fume by dotation methods.

Other objects and advantages will become readily apparent as this specification proceeds.

We have now discovered thatrupon subjecting metallurgical dusts and fumes of relatively low selenium content to a densitic-ation roast and thereafter grinding the roasted material and extracting the water-solubles therefrom, the residual material, upon appropriate conditioning, and otation provides a selenium concentrate having a significantly increased selenium content. The lot-ation product being at Ileast ten times richer in selenium than the original metallurgical dust or fume provides an excellent feed material for use in the well known selenium recovery procedures.

The metallurgical fumes and dusts referred to herein are of a varied type and character wherein the selenium content of Ythe material generally ranges from l to 5% by weight or possibly slightly higher. A principal source of such material is the lead bl-ast furnace dust collected, for example, lin an electrostatic precipitator. Although the treatment of such material is hereinafter described more ful-ly for purpose of illustration, it will be understood -that the invention is not lim-ited thereto but is also applicable to other low grade metallurgical dusts `and fumes typically containing around l to 51% selenium.

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Patented Nov. r14, 1961 llutly amorphous material to a more crystalline or denser form to make it more amenable for subsequent treatment. This is accomplished by heating said material in any suitable furnace such as la muiile or hearth toaster to -a temperature between 200 and 500 C. for a period ranging from about 10 minutes to an hour or so depending on the temperature actually used with a period of about 20 minutes sufcing at the preferred roasting temperature of about 300 1C. Air need not be excluded during the densifying roasting.

The densilied material is then ground in a ball mill or by any other appropriate means for producing material of suitable particle size preparatory to treatment thereof by froth flotation. In this connection, a grind in which about 85% Vof the material is reduced to minus 200mesh`screen size has been found to produce satisfactory notation recoveries. lPrior to actual flot-ation, however, ityis essential that thedensied material be leached with water to remove the soluble salts contained therein. f

The step of grinding the material and then extracting with Water for the removal of water-solubles may be carried out more or less simultaneously as by resorting -to wet grinding wherein the water for leaching the material is added directly to the ball mill during the grind'- ing operation. The slurry obtained thereby upon completion of the grinding may then be `iiltered or centrifuged to separate the aqueous liquid containing the water-solubles from the selenium-containing residue. The amount of Water used for leaching the comminuted material is not of particular significance, it being important, however, to extract the water-soluble salts kas completely as possible using several washes if necsary. i

Alternatively, the densiied smelter dust or fume may be leached prior to grinding with the leached calcine being then ground preparatory to its being subjected to flota-tion. For best results, however, it -is preferred to leach the material in a comminuted state since the extract-ion of the water-soluble salts is made easier `and more effective thereby. .In actual practice, the leach- Y ing operation is best eifected by acombination of both In Ia preferred embodimentV of the invention, the dust Vapproaches in that the calcine obtained from the densifying roast may tbe leached to remove a major portion of the water-soluble constituents, the remainder being thereafter extracted during comminuation of the material in the presence of water as previously mentioned. In either event, the material substantially freed of its water-soluble constituents should be made available for pulping and ensuing otation in a form suitable for the purpose. i v I That the prior treatment of smelter dusts and fumes of low selenium vcontent. is essential for enabling successful dotation for the purpose of obtaining a satisfactoryY selenium concentrate product is made readily apparent by comparative tests on batches of the Vsame feed material. :For example, all tests on raw dusts and fumes referring toA attempts to obtain a'satisfactory selenium flotation concentrates were negative as were the results using'material that was densiiied butnot leached prior to flotation'. By contrast, results on leached densitied smelter dusts and fumes proved exceedingly satisfactory in that the subsequent flotation as will be hereinafter described in detail produced highly satisfactory results.` l

`The effect of the densifying roasting and leaching on smelterV dusts Vand fumes is illustrated in the case of Cottrell dust yfromlead blast furnacing wherein, for eX- ample, a 2500 gmt sample assaying 1.25% by weight of selenium was densied at 250 C. for one hour in a gas tired furnace toyield 2040gm. of calcine still Aassaying 1.25% selenium. The calcine, after leaching with water yielded 1700 gm. of residue assayingv1.`5% selenium.

The residue was ground for l minutes in a steel ball mill with seven liters of water. Even without separating the water at this stage the resulting pulp produced by the aforesaid grinding gave very good flotation results whereas in the absence of either the densifying roasting or the leaching steps the tlotation results were negative.

Following removal of substantially all of the watersoluble content from the calcined material preferably while in a ground state, the residue is treated to provide an aqueous pulp for use in froth flotation. It has been found that froth flotation is best carried out on a conditioned yaqueous pulp containing from to 40% and preferably about solids, t-he mixture being made slightly acidic by the addition of sulfuric acid to provide a pH between 5.0 and 6.8 and preferably about 6.0. At appreciably lower pH values, lthe recovery of selenium has been found to be materially reduced. It has also been found that the conditioning period of the reagentized pulp prior to actual flotation also materially affects the results obtained depending somewhat on the nature of the material being processed. Conditioning periods of from 5 to 30 minutes lgenerally are satisfactory in the usual case, afterV which a frother reagent is added if omitted during the conditioning step. Thereafter, otation is carried out in the usual manner in any desired type of convensional froth flotation cell.

Although collectors and frothers commonly used in froth flotation concentration of metalliferous ores may be used for obtaining the desired selenium concentrate product, with the collector reagents generally consisting of amines, fatty acids, Xanthates and the like and the frother reagents consisting of substances such as pine oil, alcohols, phenols, cresols, glycols, glycol ethers and the like, it has been found that a Xanthate collector used in the amount of about 0.05 to 0.5'0 lb. per ton of leached residue feed and glycol ether type frother used in the amount of about 0.05 to 0.20' lb. per ton of leached residue feed provide the best results. Of the Xanthates which are commonly used in the form of alkali metal salts including substances such as sodium ethyl Xanthate, potassium amy-l Xanthate, sodium heXyl Xanthate, sodium amyl Xanthate and the like, the product potassium hexyl Xanthate known commercially as Z-lO has proven to be very effective as a collector reagent. In some instances, the consumption of collector reagent may be .decreased and the recovery of selenium in the concentrate increased somewhat by using kerosene or an equivalent substance in conjunction with the Xanthate collector reagent. With respect to the ,frother reagents, the product known commercially as Dowfroth 250 which is understood to be a polypropylene glycol ether has been found to be highly satisfactory.

The otation of the conditioned pulp is carried out in an entirely conventional manner with the frother being -added in the indicated proportion either to the conditioner or upon introduction of the conditioned pulp to the flotation cells. Flotation may be carried out in as many stages as desired but usually one cleaning of the roulgher concentrate without additional reagents suffices to Vgive a desirable product without incurring undue losses of selenium to middlings. The froth product from the cells generally contains from about 12 to 20% by Iweight of selenium, the recovery of selenium usually amounting to better than 60%. Cadmium, if present in the feed material, is also recovered as a component of the concentrate product in amounts corresponding to better `than 50-60% of the quantity present in the leached residues. Arepresentative concentrate product obtained from lead blast furnace dusts and fumes assaying 1.6% by weight of selenium and 2.6% by weight of cadmium after roasting and leaching assayed at 16% selenium and 24.6% cadmium following froth flotation as hereinbefore described.

The process comprising the present invention as discussed above is illustrated in the accompanying drawing wherein the various operations are Vschematically indicated. Although the grinding and leaching steps are shown as separate and distinct steps in the ow sheet, it will be understood these steps can be combined in one operation as previously stated.

While lin the foregoing specification this invention has been described in relation to a preferred embodiment thereof and specific details of this embodiment havev been set forth for the purpose of illustration, it will be apparent to those skilled in the art that this invention is susceptible to other embodiments and that many of the details herein set forth can be varied considerably without departing from the basic concept of the invention.

What is claimed is:

1. In a process for the recovery of a selenium concentrate from metallurgical dusts and fumes containing selenium in relatively small quantities generally belowv 5% by weight, the steps ycomprising roasting said dusts and lfumes to provide a densiied calcine, grinding said calcine, leaching the ground calcine with water to remove substantially all the water-soluble content therefrom, pulping said ground and leached residue in water, acidif-ying the resulting mixture to a pH between 5.0 and .6.8, reagentizing and conditioning fthe mixture, subjecting the conditioned mixture to froth otation and recovering a selenium-enriched fraction therefrom.

2. The process of claim 1 wherein the roasting operation is carried out at a temperature of about 300 to 450 C. for a period of about 20 minutes.

3. The process of claim l wherein prior to froth flotation the ground and leached residue is pulped in water to a solids content from about 25 to 35%.

4. In a process for the recovery of a selenium concentrate from metallurgical dusts and fumes containing selenium in relatively small quantities generally below 5'% by weight, the steps comprising roasting said dusts and fumes to provide a densied calcine, leaching said calcine to remove water-soluble materials therefrom, grinding the leached residue, pulping the ground and leached residue in water, acidifying the resulting mixture to a pH between 5.0 and 6.8, reagentizing and vconditioning the mixture, subjecting the conditioned mixture to froth otation and recovering a selenium-enriched fraction therefrom.

5. The process of concentrating the selenium contained in a metallurgical dust from smelting of lead ores, said dust generally containing below 5% by weight of selenium comprising the steps of roasting said dust for from 10 minutes to an hour at a temperature ranging from 200 to 450 C., grinding the resulting calcine, leaching the yground material with water to effect removal of at least a major portion of the water-soluble content therefrom, pulping the resulting ground residue in an aqueous medium to a solids content of from 25 to 35% Said aqueous medium being acidied to a pH between 5.0 and 6.8, reagentizing the acidied aqueous pulp with a Xanthate collector, conditioning the reagentized pulp for from 10 to 30 minutes, subjecting the conditioned pulp in the presence of a frother reagent to flotation and recovering selenium-enriched concentrate.

6. The process of claim 5 wherein the grinding and leaching steps are carried on simultaneously.

7. The process of claim 5 wherein the frother reagent is a polypropylene glycol ether.

8. The process of claim 5' wherein the xanthate collector is potassium hexyl xanthate.

9. The process of claim 5 wherein kerosene is used in conjunction with the Xanthate collector.

References Cited in the file of this patent Gmelin: Handbuch der anorganischen Chemie, System No. 10, Selenium, 8th ed., 1949, pages 78-95, page 86 especially relied on.

J. Perrys Chemical Engineers Handbook, 3rd ed., McGraw-Hill Book Co., Inc., New York, NY., 1950, page 1088. 

1. IN A PROCESS FOR THE RECOVERY OF A SELENIUM CONCENTRATE FROM METALLURGICAL DUSTS AND FUMES CONTAINING SELENIUM IN RELATIVELY SMALL QUANTITIES GENERALLY BELOW 5% BY WEIGHT, THE STEPS COMPRISING ROASTING SAID DUSTS AND FUMES TO PROVIDE A DENSIFIED CALCINE, GRINDING SAID CALCINE, LEACHING THE GROUND CALCINE WITH WATER TO REMOVE SUBSTANTIALLY ALL THE WATER-SOLUBLE CONTENT THEREFROM, PULPING SAID GROUND AND LEACHED RESIDUE IN WATER, ACIDIFYING THE RESULTING MIXTURE TO A PH BETWEEN 5.0 AND 6.8, REAGENTIZING AND CONDITIONING THE MIXTURE, SUBJECTING THE CONDITIONED MIXTURE TO FROTH FLOTATION AND RECOVERING A SELENIUM-ENRICHED FRACTION THEREFROM. 